The present invention relates in general to the field of synchronous generators, and more particularly, to an apparatus and method for protecting synchronous generators from off-nominal frequency deviation coincident with alternating forces excitation. The present invention has particular application when used with generators within a power generator plant integrated into a power transmission grid system.
Many power generation plants produce electricity by converting potential energy (e.g. fossil fuel) into mechanical energy (e.g. rotation of a turbine shaft), and then converting the mechanical energy into electrical energy (e.g. by the principles of electromagnetic induction). These power generation plants typically use a turbine to convert the potential energy into mechanical energy and a generator to convert the mechanical energy into electricity.
The electricity is then distributed from the power generation plants to consumers through power transmission grid systems, such as those extending throughout the United States and portions thereof. These power grids operate at a predefined nominal frequency, such as 60 Hz in the United States and 50 Hz in Europe. This nominal frequency typically deviates by about +/xe2x88x920-5 Hz during normal power grid operation due to a variety or reasons such as peak demand, low electricity output, inadvertent loss of load, tripping of generators or lines, and power grid operator selection. Individual power generation plants may be required to operate at frequencies that deviate by about +/xe2x88x920-5 Hz from the power grid""s nominal operating frequency (hereinafter xe2x80x9coff-nominal frequencyxe2x80x9d) to help compensate for line loads and other reasons.
One aspect of generator design involves the accommodation of typical off-nominal frequencies at which the generator may be required to operate. As part of such accommodative design, an off-nominal frequency relay is typically used to trip the generator to an off-line mode if the generator""s off-frequency operation exceeds certain parameters that can otherwise lead to generator damage. For example, during underfrequency, it is possible to overexcite the core of the generator due to excessive Volts/Hertz excitation. This could lead to shorted laminations in the stator core. Also, since most generators are ventilated by a shaft-mounted blower and since some generators are excited by a shaft-mounted exciter, reductions in system frequency that lead to reduced speed of rotation may cause overheating of the generator or exciter. Additionally, operation at speeds other than nominal may excite mechanical resonances of generator components, leading to vibration and cyclic fatigue. In particular, generator shafts often have torsional natural frequencies that are close to twice normal operating frequency, so operation at off-nominal frequencies coincident with high levels of local negative sequence excitation (which causes a double frequency torque to be applied to the rotor) may cause torsional fatigue of rotating components.
Another aspect of generator design involves the accommodation of typical internal electrically based alternating forces excitation, such as negative sequence excitation and system harmonics, such as may be generated by electronic loads, arc furnaces, and HVDC power terminals. As part of such accommodative design, an negative sequence relay is typically used to alarm the operator or trip the generator to an off-line mode if the generator""s negative sequence excitation exceeds certain parameters that can otherwise lead to generator damage. Negative sequence currents in the generator cause a double-frequency torque on the generator rotor and cause eddy currents to flow on the rotor surface, which could result in excessive heating and, in some cases, arcing. In recognition of this, generator standards provide parameters for expected amounts or levels of negative sequence currents for design (typically 6 to 10% of rated current). It is believed that these parameters were established based primarily on thermal considerations with the understanding that the system frequency would typically remain within a small bandwidth (e.g. about +/xe2x88x921-2 Hz) of the power grid""s nominal frequency. If the generator operates at frequencies outside this protective bandwidth while it is receiving negative sequence excitation, the generator may become susceptible to significant mechanical response and electrical eddy current damage. Mechanical resonance is caused by a natural electrical system frequency coinciding with the torsional natural frequency of the rotor shaft, which can cause fatigue damage. Although expensive damping systems can be used to mitigate resonance, such systems can introduce other operating difficulties that may negatively impact generator operation, possibly resulting in damage to the turbine and/or generator. Although off-nominal frequency operation and alternating forces excitation are independent events and the likelihood of coincidental simultaneous occurrence of these two events is small, such simultaneous occurrence is nevertheless possible. Normally, off-nominal frequency operation is system-wide, while alternate forces excitation is local in origin, typically caused by open transmission lines or failure of certain pieces of equipment. Heretofore, generator design did not generally account for the simultaneous occurrence of these two phenomena.
There is thus a need to for an apparatus and method for protecting synchronous generators against off-nominal frequency deviation and alternating forces excitation, and a synchronous generator that improves upon the prior art.
The present invention helps protect synchronous generators against off-nominal frequency deviation and alternating forces excitation. The present invention recognizes that linking off-nominal frequency operation with alternating forces excitation protection can provide several benefits, such as superior generator protection and an increase in the allowable operating frequency range of synchronous generators on a power grid. The present invention also provides a synchronous generator that improves upon the prior art.
One aspect of the present invention thus involves a linked electrical relay system adapted for use in a synchronous generator; the system comprising a electrical relay system in operative association with an electrical signal representative of an actual frequency at which the generator is operating and to an electrical signal representative of an actual forcing amount at which the generator is operating, the relay system adapted to respond to the actual frequency signal and the actual forcing signal, and wherein the relay system compares the actual frequency signal with a predetermined desired frequency range and compares the actual forcing signal with at least one predetermined forcing amount, and selectively alarms the operator or trips the generator to an off-line mode depending upon the comparisons.
Another aspect of the present invention involves a method for operating a synchronous generator, the method comprising; providing a relay system adapted to respond to an electrical signal representative of an actual frequency at which the generator is operating and to an electrical signal representative of an actual forcing amount at which the generator is operating; providing a desired off-nominal frequency range, a desired lower maximum forcing amount, and a desired upper maximum forcing amount; measuring an actual frequency at which the generator is operating, and measuring an actual forcing amount at which the generator is operating; comparing the actual frequency with the off-nominal frequency range, and comparing the actual forcing amount with the lower and/or upper forcing amounts; and selectively alarming the operator or tripping the generator to an off-line mode based upon the frequency comparison and the forcing comparison.
Yet another aspect of the present invention involves a synchronous generator, comprising; an axially extending rotor enclosed in an annular stator that surrounds and sleeves the rotor; a frequency source signal in operative association with the rotor and stator representative of an actual frequency at which the generator is operating; a forcing source signal in operative association with the rotor and stator representative of an actual forcing amount at which the generator is operating; a relay system adapted to respond to the frequency source signal and the forcing source signal by comparing the actual frequency with a predetermined desired frequency range and comparing the actual forcing amount with at least one predetermined maximum forcing amount, and selectively alarming the operator or tripping the generator to an off-line mode depending upon the comparisons.
Further aspects, features and advantages of the present invention will become apparent from the drawings and detailed description of the preferred embodiments that follow.